Self heating and other thermal management challenges have long existed for BJTs and HBTs. These thermal management issues have become increasingly important as BJT and HBT transistors decrease in size and performance increases. As an example of how thermal issues affect performance, conventional InP HBTs have been shown to have a 0.5 GHz FT reduction per ° C. of junction temperature rise.
T. Arai et al, “Proposal of Buried Metal Heterojunction Bipolar Transistor and Fabrication of HBT with Buried Tungsten”, 1999 International Conference on InP Phosphide and Related Materials, Proceedings, pp. 183-6, and T. Arai et al, “CBC Reduction in GaInAs/InP Buried Metal Heterojunction Bipolar Transistor”, 2000 International Conference on InP Phosphide and Related Materials, Proceedings, pp. 254-7, which are incorporated herein by reference, describe an approach to thermal management. In that approach thin fingers of tungsten are deposited on an InP substrate and the required epitaxial layers are grown on top of these tungsten fingers. When applied to HBTs the resulting device is called a buried metal HBT (BM-HBT).
The Arai method requires high quality epitaxial layers to be grown on an uneven, mixed composition (tungsten and InP) surface. This method is unlikely to ever produce epitaxial films of comparable quality to those grown only on an InP substrate, which also negatively affects reliability and performance.
This approach also requires that a metal resistant to the high temperatures needed for materials growth and a metal able to alloy to InP and its related materials must be selected. This fundamentally limits the metals compatible with the Arai method, and those metals are mostly refractory. Although tungsten has a fairly high thermal conductivity (170 W/mK) and fairly low electrical resistivity (5 μΩcm), it is not as thermally conductive or as low resistance as gold or aluminum (>300 W/mK and <3 μΩcm).
The tungsten fingers in the Arai method must also be finely patterned and comparable to the HBT dimensions to ensure proper materials growth and device fabrication. This places an additional demand on device fabrication and fundamentally limits the lateral sheet resistance of the buried metal sub-collector.
What is needed are apparatus and methods to mitigate the self-heating and other thermal management challenges associated with BJTs and HBTs. The embodiments of the present disclosure answer these and other needs.